What Is Linux

Linux is a freely available, open source, Unix-like operating
system. Written originally for the PC by Linus Torvalds, with the help
of many other developers across the internet, Linux now runs on multiple
hardware platforms, from the smallest to the largest, and serves a
wide variety of needs from servers to movie-making to running
businesses to user desktops.

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Linux has been popular for years with system administrators who are responsible for running large networks and servers because of its robustness, scalability, and flexibility. For a long time, it was seen as a geek's system--too complicated for ordinary folks. But Linux has matured, and with today's desktop environments like KDE and GNOME, and new user-friendly installations, Linux is finally coming into its own as a desktop system as well.

Linus Torvalds was a college student in Finland in 1991 when he began work on Linux as a hobby, intending to build a Unix-compatible system for his PC. By making the source code freely available, while retaining ultimate control, he opened development to other programmers. Today's Linux kernel is the result of a joint effort by thousands of programmers from around the world, working together over the internet.

What Linux Is

The name Linux is used in three different ways. First, Linux is the kernel, the heart of the operating system. Strictly speaking, this is the true meaning of Linux. The kernel sits at the lowest level and manages the hardware. If a running program wants to interact with the hardware, for example to print a document, it doesn't communicate directly with the printer, but with the kernel, which then manages the printer. In addition to managing peripheral devices like the printer, keyboard, and mouse, the kernel controls the hard drive, memory usage, concurrent program execution, networking, and system security.

Linux also refers to the operating system. The kernel alone isn't enough to provide a functional computer system; it provides the foundation, and the operating system adds the tools to make the system usable. As an operating system, Linux consists of the kernel, plus an extensive set of libraries, compilers and debuggers, system utilities and programs, as well as one or more command shells. In other words, the operating system provides the tools for programming and for managing the system. Most of the tools were developed by the GNU project of the Free Software Foundation (FSF), and they provide enhanced versions of the traditional Unix equivalents. Some others came from BSD Unix, developed originally at the University of California at Berkeley.

Because so many of the Linux programs and commands come from the FSF's GNU project, some people--most notably the FSF itself--believe that Linux should be called GNU/Linux. For the most part, people still call it Linux, but the GNU/Linux name is used for the Debian distribution, developed by the FSF and called Debian GNU/Linux.

Finally, the term Linux is used for a Linux distribution. We'll talk more about distributions later, but for now, a distribution is a combination of the kernel, the operating system utilities, and a very large set of application programs. A distribution generally includes tools for installing Linux, graphical desktop environments like GNOME or KDE, office suites, web browsers, configuration tools, and much more.

This multiple meaning of Linux can be confusing. Technically, Linux is the kernel. However, in common usage, it most often refers to a distribution. If someone says she is installing Linux, she is most likely talking about installing one of the available distributions. On the other hand, the Linux that Linus manages and controls development of is the Linux kernel. For more information on the kernel, see the Linux Kernel Archives web page.

Linux Is Not Unix

Linux is frequently referred to as a Unix-like system. We cannot call Linux a Unix system because it has not passed the tests and certifications required by The Open Group to be officially called Unix. Practically speaking, however, Linux is functionally similar to Unix; it was designed to work like Unix and for most purposes it has accomplished that goal. If you know Unix, you'll be comfortable working with Linux; if you want to learn Unix, a good way to do that is to install and work with Linux. If you come from the Windows or Macintosh worlds and don't know either Linux or Unix, the Linux desktop environments KDE and GNOME provide a familiar interface for learning, while making it easy to become productive quickly.

Features

Linux is described variously as free and as open source. The two terms have much the same meaning. As it is used to describe Linux, the term free does not mean that there is no cost to purchase the software (although it is true that many versions are available for a free, no-strings-attached download from the internet). Free is used here in the FSF sense of software freedom. From the FSF website: "The Free Software Foundation . . . is dedicated to promoting computer users' rights to use, study, copy, modify, and redistribute computer programs."

The term open source has much the same meaning. It was established to provide a term that was more neutral and didn't have the wild-eyed radical overtones that many businesses in particular read into the FSF concept of software freedom.

Whichever term you use, the major feature of open source software, including Linux, is that the source code is freely available to read or to modify. It can be redistributed with or without modifications; if you do make changes and redistribute the code, you have to make the source code for your modifications available, and you can't add restrictions on anyone else's right to freely update or redistribute the code. The licensing terms that stipulate these requirements are part of the GNU General Public License (GPL), under which Linux is distributed. An additional requirement of the GPL is that any modifications or works derived from GPL'd software must also be licensed under the GPL.